Geoscience Reference
In-Depth Information
this interface and develop a perched water table and fully saturated conditions, although
deeper layers may remain partially saturated. In several experimental studies it has
been observed that such layers can be effective enough to be a major, and sometimes
even the main transport medium for stormflow. As noted above, this type of flow was
observed to occur by Bonnell and Gilmour (1978), in conjunction with saturation over-
land flow, in a catchment in Queensland. The chemical signatures of hillslope waters in
a catchment in Wales made also Chappell
et al
. (1990) conclude that this can indeed
be the dominant mechanism for water and ion transport to lower near-stream riparian
zones. Similarly, Jenkins
et al
. (1994) used natural tracers to characterize rain water,
soil water and ground water in a moorland catchment in northeast Scotland. The inter-
face between mineral soil layers and the upper organic layers of peaty podsol were
identified as preferential pathways. Flow of water inthis upper layer was observed to
be triggered nearly instantaneously by the onset of the rain, and also to stop nearly
as suddenly as the rain ceased; the water inthis layer had a chemistry very similar
to that of the rain. In the runoff hydrograph, the peak flow was found to be domi-
nated by rain and soil water, whereas the recession part was dominated by pre-event
groundwater.
Although they did not consider this type of flow as being representative of the entire
catchment, McDonnell
et al
. (1991b) did observe it on small portions of the Maimai
catchments in New Zealand. Duringarain storm event of some 47 mm and with the
soil water suctions initially ranging between
H
60 and 150 cm water column, most
of the water was seen to flow out from the organicsoil layer perched on the mineral soil
profile; all the while, the lower soil profile remained only partly saturated. More recently,
from an experimental study on seven nested (from 8 to 161 ha) forest catchments in the
Catskill Mountains of New York, Brown
et al
. (1999) concluded that a large fraction of
the rapid delivery to the stream took place through this same mechanism. Event water
appeared to be most prevalent in the stormflow especially during dry conditions, with
relative contributions between 50% and 62% near peak flow.
=
Wavelike mobilization of the water table
As illustrated inFigures 8.5, 8.6 and 8.7, for most soils within the nearly saturated
capillary fringe, a small change in water content can result in a relatively large change in
pore water pressure. This has led to the view that the addition of a very small amount of
water to a relatively moist soil can raise the water table rapidly, almost as a pressure wave
type of propagation, to produce a saturated soil profile. Wherever the profile becomes
fully saturated this way, subsurface flow may emerge and saturation excess overland
flow is also bound to occur. This type of water table rise may be especially fast in the
lower parts of the hillslope and may result in the build up of an emerging groundwater
mound, exhibiting greatly increased hydraulic gradients and groundwater discharge to
the channel, and forming a partial or variable source area producing saturation excess
overland flow as well. Thus the phenomenon is not unlike that depicted inFigure 11.3,
except that here the water table rise is presumed to involve very little actual water
movement.
